// Copyright (c) 2011 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "base/lazy_instance.h"

#include "base/at_exit.h"
#include "base/atomicops.h"
#include "base/threading/platform_thread.h"

namespace base {
namespace internal {

    // TODO(joth): This function could be shared with Singleton, in place of its
    // WaitForInstance() call.
    bool NeedsLazyInstance(subtle::AtomicWord* state)
    {
        // Try to create the instance, if we're the first, will go from 0 to
        // kLazyInstanceStateCreating, otherwise we've already been beaten here.
        // The memory access has no memory ordering as state 0 and
        // kLazyInstanceStateCreating have no associated data (memory barriers are
        // all about ordering of memory accesses to *associated* data).
        if (subtle::NoBarrier_CompareAndSwap(state, 0,
                kLazyInstanceStateCreating)
            == 0)
            // Caller must create instance
            return true;

        // It's either in the process of being created, or already created. Spin.
        // The load has acquire memory ordering as a thread which sees
        // state_ == STATE_CREATED needs to acquire visibility over
        // the associated data (buf_). Pairing Release_Store is in
        // CompleteLazyInstance().
        while (subtle::Acquire_Load(state) == kLazyInstanceStateCreating) {
            PlatformThread::YieldCurrentThread();
        }
        // Someone else created the instance.
        return false;
    }

    void CompleteLazyInstance(subtle::AtomicWord* state,
        subtle::AtomicWord new_instance,
        void* lazy_instance,
        void (*dtor)(void*))
    {
        // Instance is created, go from CREATING to CREATED.
        // Releases visibility over private_buf_ to readers. Pairing Acquire_Load's
        // are in NeedsInstance() and Pointer().
        subtle::Release_Store(state, new_instance);

        // Make sure that the lazily instantiated object will get destroyed at exit.
        if (dtor)
            AtExitManager::RegisterCallback(dtor, lazy_instance);
    }

} // namespace internal
} // namespace base
